This invention relates to automated control systems for controlling the operation of heavy machine tools such as mills, presses and the like, used to machine a workpiece such as metal stock into predetermined forms. In many applications, it is essential for these operations to be performed in accordance with very close tolerances. This requires some means of moving and positioning very accurately the machine tool implement, i.e., a spindle carrying a mill, cutter, drill or other tool implement. For many years, a manual system was employed wherein the operator manipulated a hand crank, which in turn actuated a hydraulic valve to control the hydraulic pressure to the hydraulic ram which in turn caused the machine tool carriage to move along a way. In a typical example, the machine tool is provided with three hand cranks, each corresponding to a degree of movement of the implement, allowing the operator to move the machine tool implement along an X, Y and Z direction. A typical example of a prior art machine is the Cincinnati 28" series vertical "Hydro-Tel" (Trademark) milling machine, manufactured by the Cincinnati Milling Machine Company, Cincinnati, Ohio.
While hydraulic positioning systems have the advantage of providing great power with relatively simple mechanisms, an operator of considerable skill is required to operate these types of machines under close tolerance work, and the time required to machine workpieces to very close tolerances dramatically increases.
To some degree, these problems are obviated by the use of a "tracer" system wherein a stylus is provided and connected to separate hydraulic valves so that simply by running a stylus along the edge of a pattern, the tool implement automatically follows the stylus' motion to reproduce the pattern. Nevertheless, this system is also subject to considerable drawbacks. To prevent gouges and marking of the workpiece, the speed at which the implement is moved should be substantially constant and as slow as possible. It requires an operator of very considerable skill to run a stylus along a pattern at a constant, slow speed to minimize gouging, scarring or marking of the workpiece, and even with a skilled operator, some marking of the workpiece is virtually inevitable. Once again, it is very difficult to reproduce the pattern within very close tolerances by the use of such a tracer system.
The technical disadvantages of the manual hydraulic control system have been largely overcome by the advent of numerically controlled or computer controlled machine tools. With such systems, as presently understood by applicants, the hydraulic mechanism is replaced with an electrical mechanism. The hydraulic ram associated with each axis of movement is replaced with a large ball screw rotated by a large electric motor precisely operated by the computer controller. Thus, the computer may be programmed in an appropriate manner to move the implement through a desired movement or plurality of movements by causing the electric motor to drive the ball screw through angular displacements at a specified speed. In such an embodiment, the computer accurately senses the angular displacement of the motor shaft and thereby very accurately determines or assumes the position of the tool implement.
Such an automated electrical system suffers the disadvantage that the electric motors and the ball screw must be of relatively large size to move the heavy machine tool carriages, and accordingly are quite expensive. Typically, it is known that a manually controlled machine such as the Cincinnati "Hydrotel" milling machine is retrofitted by the manufacturer at its factory by removing the hydraulic mechanisms and retrofitting the electric motor, ball screw, computer and associated equipment. Applicants understand that this retrofit necessitates a six-month downtime while the machine tool is shipped to the factory from its installation site, refurbished, retrofitted with the electric control system and then shipped back to its installation site. The cost is very substantial.
There are thousands of machine tools in use today which have hydraulic manual controls. A tremendous need exists for a means to fit these existing machines with automated control systems at an economical price, with a reasonable installation time, and without transporting the tool from its work site.
It is, accordingly, one object of the present invention to allow on-site retrofitting of machine tools having manual controls with automated controls at a relatively low cost and with a minimized installation time.
Another object of the present invention is to provide a mechanism for controlling a hydraulically operable machine tool to close tolerances.
It is another object of the present invention to provide an automated control system for hydraulically operable machine tools capable of machining parts to very close tolerances.
Another object of the present invention is to provide an automated control capability but to retain the capability of manual tracer operation in the event of computer failure, or for use on small production runs.
Other objects will be apparent from the following description.